#include <iostream>

using namespace std;

int main() 
{
	int k;cin>>k;
	
	int ret=0,cnt=1;
	
	while(k)
	{
		int t=cnt;
		while(k&&t)
		{
			ret+=cnt;
			k--;t--;
		}
		cnt++;
	}
	 
	cout<<ret<<endl;
	return 0;
}
